Field of the Invention
The present invention generally relates to the field of power cables for electric power transmission and distribution, in particular to cables for underwater or underground use.
Description of the Related Art
Power cables for power transmission in the Medium Voltage (MV) and High Voltage (HV) ranges (from 1 kV to 35 kV for MV and higher than 35 kV for HV) typically comprise one or more conductors (one conductor for single-phase power transmission, three conductors for 3-phase power transmission; cables with more than one conductor are also referred to as “multi-core” cables). Each conductor is usually surrounded by an inner semiconducting layer, an insulation layer and an outer semiconducting layer, the conductor and said layers being hereinafter referred to as “core”. An armor, for example of metal wires or metal tapes, can be provided to surround the core/s, especially in case of underwater cables, for providing resistance to tensile stress. In some applications, the armor may be covered by a polymeric sheath.
When a power cable of the abovementioned type has to be installed in a wet or potentially wet environment, such as underwater or underground, its core/s should be protected from water penetration that may lead to electrical breakdown. For this purpose, the cable is equipped with a water barrier, which can be provided around each core or around the bundle of cores, for preventing or at least reducing the occurrences of water trees possibly producing electrical breakdown.
Different types of water barrier are known. In particular for underwater or underground cables the material at present more frequently used for implementing water barrier is lead, especially for HV submarine cables. Lead has proved to be a reliable and sturdy sheathing material for implementing barrier capable of efficiently preventing the water ingress. However, barrier made of lead are affected by some drawbacks. First of all, water barrier made of lead are quite heavy, and this increases the production, transport, storage and deployment costs of the cable. Moreover, ecological concerns are inducing to dismiss the use of lead, because of its environmental toxicity.
Water barrier made of welded copper have been proposed, as disclosed, for example, in the spring/fall 2009 C11-Minutes of Insulated Conductors Committee (see http://www.pesicc.org/iccwebsite/subcommittees/subcom_c/C11/C11 Minutes2009.htm) and in “HVAC Power Transmission to the Gjoa Platform” by E. Eriksson et al., 8th International Conference on Insulated Power Cables, A.6.5, Jicable' 11-19-23 Jun. 2011, Versailles—France. In particular, cables are said to be provided with welded corrugated copper sheath as a protection against radial moisture penetration.
A copper sheath is considered to be very resistant against fatigue phenomena, which can demolish a lead sheath after a large number of bends. For this reason, a copper sheath can be used for underwater dynamic power cables, which are suspended freely from floating oil and gas platforms and are subject to repeated bends due to waves.
According to known solutions, water barriers made of welded copper are manufactured by wrapping a copper foil around the cable and then welding together the edges of the wrapped copper foil along the cable length.